<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
<style>
    p em:first-child{
        /* 所有p元素下，同级第一个且为em的元素 */
        color: red;
    }
</style>
</head>
<body>
    <h3>hhhhh3</h3>
    <p>
        <em>111111</em>
        <em>222222</em>
    </p>
    <div>
        <p>
            <em>111111</em>
            <em>222222</em>
        </p>
    </div>
</body>
</html>